Luo et al. (2002). A landmark paper on Cynodont phylogeny.
Rowe (1988). The argument for a crown-group definition of Mammalia.
Zheng et al. (2013.) A recent phylogeny of the late cynodonts.
Ruta et al. (2013). A recent paper that uses my preferred definition of Mammalia.
O'Leary et al. (2013). This paper argues that placental mammals did not diversify until after the dinosaur extinction at the end of the Cretaceous. (I think they're wrong, and the reasons are presented here and here.)